168
BIOLOGY OF FISHES
.
there is also an internal sympathetic system.. . The cranial
nerves are
for the most part the same, With some varia'
,
tions in their development and importance. The VIIIth
nerve, for example, is specially— large and is connected With
an independentcentre in the medulla, the tuber acusticum.
This is supplied from the general cutaneous nerves, lateral
._
line system, as well as from the auditory organ or statocyst.
.
’
Thesurface of the brain is smooth and in some places
_
simply covered With epithelium.
‘
The peculiar features of the brain in shes are the great
elongation and separation of the principal parts from one
another.
These may be considered a sign of its more
Ï
primitive condition, but it is also advisable to take them in
c0nnection With the »inode of development and the forces
'
_
acting upon the brain, It is certain that the shape of the
brain is determined by the form and capacity of the “cranial
:;
cavity and the latter is determined by the forces acting on the '
5
Skull, external pressure, balance of the body, etc.
'
'
.
This can be seen very clearly on comparing the brain of
a Dog—sh With that of a Herring (Fig. 43). The head of the
adult Dog—sh appears to be narrow and elongated, but on
examining the brain we nd that the anterior parts are.
'Ï
expanded well out to the sides to form the olfactory lobes,
_
*
whilst the whole forebrain is distinct, With…indic‘ations of
two cerebral hemispheres (fb) This 'fgreatfÿexpansion
_
Can
f
be referred backto
ofthe embryomthe €gg:
capsule and stands
marked contraStÏ_tô_Ïthe
condition'of the
;heÏHèiÿg ‘îvvhere there is no :
:.
attempt
When we
-
reÏèàll* the
the brain of the latter
Ï:
_
Ï
free-sw1mmmg mode. of life and the pressure
’
,
of
it is not difcult to.recognisè the
,
sources of theSe—gr6at. differences.
.
—
»
_
’
‘
_
In the TelecSts the optic lobes or ’mid—brain (mb) lie. just
:
in front of and above the artiCulation of the loWer jaW With
,
-
—
"
_
the skull—(sphränotic) and are thus pres‘sed upwards t0.a.morê:
'
in the ElaSmobranchs,
the .cereè_ ,
.
_
-bellumbehind(hb) is still more elevated (Fig. 43),Œhe àl-ineÎ
3'